1. Field of the Invention
The present invention relates to a method for manufacturing a ferroelectric thin film using a sol-gel process and, more particularly, to a method for manufacturing a fine ferroelectric thin film having improved crystallinity and dense microstructure as well as improved electrical properties and a preferred (predominant) orientation using a sol-gel process.
2. Description of the Related Art
In a sol-gel process, hydrolysis or poly-condensation is carried out in a solution made of an inorganic metal compound so that a sol is solidified into a gel. The gel is then heated, thereby producing an oxide solid. In the sol-gel process, the solution is turned into a sol having minute grains dissolved therein by hydrolysis or polymerization of organo- or inorganometallic compound of a metal and further reacted for gelation. The resultant porous gel is then heated to produce amorphous, glass and polycrystalline structures. When producing the polycrystalline structures, because the initially formed gel is largely amorphous, further processing is necessary to achieve and produce the polycrystalline structures.
The sol-gel process is advantageous in several aspects: finely sintered polycrystalline ceramics can be generated at low temperature, high homogeneity can be attained, and new glass and ceramic compositions that cannot be produced using general techniques are achievable. The sol-gel process also allows polycrystalline ceramics made of fine, homogeneous grains to be synthesized. A further benefit of the sol-gel process is the high level of manufacturing efficiency that can be realized as compared with traditional sputtering or chemical vapor deposition (CVD) processes. By virtue of these advantages, the sol-gel process is widely applied in fields relating to the manufacture of compositions, fine structures, fibers and thin films.
Ferroelectric raw materials exhibit spontaneous polarization and cause polarization inversion due to an external electric field. Ferroelectric random access memories possess the most advantageous characteristics for information storage among the presently known nonvolatile memories. Ferroelectric raw materials for use in ferroelectric random access memories require characteristics such as large amounts of polarization (switched charge amounts), small dielectric constants, good resistance to polarization fatigue, good memory retention, high speed in polarization switching, and low leakage currents. A representative example of ferroelectric raw materials used in ferroelectric random access memories are PZT (Pb(Zr, Ti)O3) and PLZT (Pb(La, Zn)TiO3), both of which have a large amount of polarization and a relatively small dielectric constant.
Generally, methods for forming oxide thin films of composites are classified as vapor phase methods such as sputtering and CVD, and liquid phase methods such as the sol-gel processing. The vapor phase methods can form uniform films, but devices made using vapor phase methods are costly and generally suffer from low productivity as well as problems relating to unstable film characteristics.
For sol-gel processing, however, while it is easy to control the film composition, it is difficult to form uniform films due to a tendency for a secondary phase to form in the film as well as the tendency of the film to powder. Another problem with the conventional sol-gel technique is that striped patterns are produced due to a difference in the concentration between local parts due to the presence of long sol-gel chains in the solution.
A feature of the present invention is to provide a ferroelectric thin film having excellent crystallinity and dense microstructure, and which can eliminate a local difference in the concentration of the film, which is a primary cause of striped patterns generated during the manufacture of the thin film, by shortening the length of a sol-gel chain in a solution and improving uniformity of the film.
In a preferred embodiment of the present invention, there is provided a method for manufacturing a ferroelectric thin film using a sol-gel process, the method comprising the steps of dissolving a Pb precursor using a solvent to prepare a Pb solution and stabilizing a Zr precursor and a Ti precursor to prepare a Zr solution and a Ti solution, respectively, mixing the Zr solution and Ti solution, stirring the Tixe2x80x94Zr mixed solution with the Pb solution, hydrolyzing the Tixe2x80x94Zrxe2x80x94Pb mixed solution to prepare a ferroelectric solution, and forming a ferroelectric thin film on a substrate using the ferroelectric solution.
The ferroelectric thin film may be defined by the general formula Pb(ZrxTi1xe2x88x92x)O3 where (0 less than x less than 1).
As a Pb precursor, PbO or Pb(OAc)2.3H2O is preferably used. As a Zr precursor, Zn(n-OPr)4, Zn(n-OBt)4 or other alkoxide-based compounds can be used. As a Ti precursor, Ti(i-OPr)4, Ti(i-OPr)2, Ti(i-OPr)2(acetylacetonate)2, Ti(i-OBt)4 and other alkoxide-based compounds can be used.
In the present invention, preferred solvents for stabilizing the Zr precursor include acetic acid, n-propanol and 2,4-pentanedionate. Preferred solvents for stabilizing the Ti precursor include 2,4-pentanedionate and n-propanol. Preferred solvents for dissolving the Pb precursor include acetic acid and alcohol.
The step of forming the ferroelectric thin film may include the sub-steps of coating the ferroelectric solution on the substrate, and subjecting the coated substrate to heat treatment. The ferrolelectric solution is prefereably spin-coated on the substrate.
The step of forming the ferroelectric thin film may include repeating a predetermined number of times the sub-steps of coating the ferroelectric solution on the substrate and subjecting the coated substrate to heat treatment, and then performing annealing for pre-treatment or performing rapid temperature annealing after the predetermined number of times repetition.
In an alternate embodiment of the present invention, there is provided a method for manufacturing a ferroelectric thin film using a sol-gel process, comprising the steps of dissolving a Pb precursor using a solvent to prepare a Pb solution and stabilizing a Ti precursor to prepare a Ti solution, stirring the Ti solution with the Pb solution, hydrolyzing the Tixe2x80x94Pb mixed solution to prepare a ferroelectric solution, and forming a ferroelectric thin film on a substrate using the ferroelectric solution.
The step of forming the ferroelectric thin film may include the sub-steps of coating the ferroelectric solution on the substrate, and subjecting the coated substrate to heat treatment.
These and other features and advantages of the present invention will be readily apparent to those of ordinary skill in the art upon review of the detailed description that follows.